1. Field of the Invention
The present invention generally relates to the art of optical communications, and more specifically to an electro-optic modulator configuration and method which enable modulation of an optical carrier with electrical signals having extremely high frequencies.
2. Description of the Related Art
Transmission of signals using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is generally modulated with an optical signal, and propagates to a remote receiver either directly through the atmosphere, or via a system of optical fibers and repeaters. The maximum frequency with which an optical carrier or light beam can be modulated determines the bandwidth limitation of an optical communications system. At the present state of the art, generation and transmission of extremely high frequency (EHF) microwave signals in the millimeter wave frequency band of approximately 20 to 100 GHz has not been attainable in practical applications. Semiconductor diode lasers, which are suitable for generating optical carriers for fiber-optic communications, have not been capable of modulation with EHF frequencies at the present state of the art. Examples of attempts at high frequency modulation of semiconductor diode lasers is documented in an article entitled "Intermodulation distortion in a directly modulated semiconductor injection laser", by K. Lau, Applied Physics Letters vol. 45, no. 10, pp. 1034-1036, (Nov. 15, 1984), and further in an article entitled "Indirect Subharmonic Optical Injection Locking of a Millimeter-Wave IMPATT Oscillator", by P. Herczfeld, in IEEE Transactions on Microwave Theory and Techniques, vol. MTT-34, no. 12, pp. 1371-1375, (Dec. 1976). However, this approach has low efficiency, produces modulated signals with a weak harmonic content (less than -20 dBc), and also substantially reduces the operating lifetime of the laser.